Application of anaerobic biological treatment for sulphateremoval in viscose industry wastewater

V Parravicini K Svardal H Kroiss

Institute for Water Quality Resources and Waste Management Vienna University of Technology1040 Vienna Karlsplatz 13226 Austria EuropeEmail vparravi@iwagtuwienacat

Abstract Long term labscale and benchscale experiments were performed to investigate the feasibility of

the anaerobic process to treat wastewater from a pulp and viscose fibre industry Anaerobic wastewater

treatment enables an advantageous combination of COD sulphate and zinc removal from viscose

wastewater The aim of the investigations was to evaluate the influente of the free sulphide concentration

on COD and sulphate removal efficiency and on the substrate competition between sulphate reducing and

methanogenic bacteria Since the wastewater did not contain enough COD for complete sulphate removal

it was of main interest to determine favourable process conditions to steer the substrate competition in

favour of sulphate reduction Further experiments in benchscale permitted to evaluate applicable COD

loading rates and gain fundamental information about process stability and optimization for largescaleimplementation The present work will deal with the most relevant experimental results achieved and with

important technological aspects of anaerobic treatment of viscose wastewater

Keywords anaerobic wastewater treatment hydrogen sulphide inhibition sulphate reductionviscose wastewater

Introduction

The wastewater generated by the viscose production process is generally characterised by a highconcentration of sulphate and zinc and mediam strength of organic carbonaceous matter Sulphate does

not represent a direct threat for the aquatic environment because of its low toxicity and reactivity For this

reason sulphate emissions in wastewater generally do not undergo strict regulations In Austria howeversulphate concentration in the receiving water bodies is regulated by limit values AImVF 1994 In this

way direct inhibiting effects of high sulphate concentrations on aquatic biological processes as well as

indirect problems caused by the formation of hydrogen sulphide can be prevented Under anaerobic

conditions in the natural environment sulphate reducing bacteria SRB use sulphate as terminal electron

acceptor for the degradation of organic matter HzS which results from dissimilatory sulphate reduction is

toxic malodorant corrosive and can cause severe damages eg to concrete structures

During anaerobic treatment of sulphate rich wastewater SRB compete with methanogenic bacteria

MB for the available organic substrate MB and SRB both utilized as substrate hydrogen and acetatethe main intermediate products of the anaerobic degradation of organic matter

Methanoegnesis 4Hz COz CH4 2Hz0 4G 1307

CH3000 H2O CH4 HC03 4G 31

Sulphate reduction 4H2 5042 H HS 4H2O 4G 152

CH3000 5042 HS 2HC03 4G 476

4G Free enthalpy in kJmol by 25C 1 atm andpH 7 calculated according to Thauer et al 1977

Substrate competition will play a significant role during anaerobic treatment of viscose wastewaterSince in this case the COD source for sulphate reduction is limited Considering the thermodynamic egfree enthalpy 4G and kinetic eg specific growth rate n substrate affinity ks proprieties of SRB it

can be expected that sulphate reduction will outcompete methanogenic consortia Several studies on

anaerobic reactors at different scale have already confirmed the predominante of SRB on substrates like

hydrogen propionate and butyrate with pare or mixed culture eg Stams et al 2004 Colleran et al1995 On the contrary substrate competition seems to be very intensive during the mesophilicdegradation of acetate Omil et al 1998 Visser et al 1993 and methanol Weijma 2000 Greben et al2000 In addition to growth kinetics also many other factors like the type of seed sludge HzS inhibition

pHvalue and temperature were found to influente the outcome of the competition between SRB and MB

Hulshoff Pol et al 1998Anaerobic treatment of viscose wastewater has already been studied in previous research works in lab

scale thus with emphasis on zinc remova Kroiss et al 1985 The HZS produced by sulphate reduction

precipitates zinc ions as zinc sulphide

Zn2 5042 HZS ZnS HZSO4

As zinc sulphide has a lower solubility than zinc hydroxide sulphate reduction would be advantageousin regards to effluent concentrations as compared to the frequent applied zinc precipitation with lime

Furthermore zinc precipitation as zinc sulphide opens new challenges for the reuse of zinc in the viscose

productionThus anaerobic treatment of viscose wastewater allows an advantageous combination of COD

sulphate and zinc remova Due to viscose wastewater composition HZS production during anaerobic

treatment aiming for sulphate remova will exceed the HzS needed for Zn precipitation For this reason a

further treatment step for HzS elimination has to be foreseen for effective sulphate remova Excess HzShas to be removed from the anaerobic reactor on one hand to avoid its reoxidation to sulphate duringaerobic posttreatment on the other hand to reduce its inhibiting effect on SRB HzS inhibition is

detrimental for process efficiency and can leal to process failure Kroiss PlahlWabnegg 1983Considering that HzS will be distributed in the anaerobic reactor over the gas phase and the liquid phaseaccording to the HenryDaltons law

HZSIq Kg HZSgas

HZSiq HzS concentration in the liquid phase moUm3HZSgag HzS parcial pressure in the gas phase barKH Henrys constant depending on temperature moUm3bar

HzS remova can proceed either in the gas or in the liquid stream Figures la and lb For this purpose

severa technologies based on chemicalphysical or biological HZS remova processes Claus process

biological sulphide oxidation etc have already been applied in large scale Janssen et al 2000 HZS is

recovered in the most cases as elemental sulphur and aecording to the purity obtained it can be reused in

different applications eg fertiliser industry sulphuric acid production

a HZS and heavy metal ions remova in the gas phase

HZSrich gas recycling loop

effluent

influentmetal ions sulphate reduction HZSremova

wastewaterprecipitation gas scrubbing

zinc

HZSfree gas recycling loop

b HZS and heavy metal ions remova in the water phase

HZSrich water recycling loop

effluent

influent

wastewatermetal ions sulphate reduction sulphide remova

precipitation

1zinc sludge

sulphur sludge

i

HZSfree water recycling loop

Figure 1 Two examples of process configurations integrating sulphate COD and heavy metais remova

in industrial wastewaters Cens et al 2000

In the liquid phase depending on the pHvalue HzSiq will be present in the unionised free form

HzSfree and as ion HS and Sz Helgeson 1967

2H2Sfree H H HS H H S kaissc 117X10 kazssc 109X1012

In the usual pHrange applied during anaerobic treatment the Concentration of ions Sz can be

neglected According to the current knowledge HZS inhibition is related to HzSfree Concentration as onlyunionised HZSmolucules can pass the cell membrane by diffusion Speece 1983 HZS inhibition is

therefore strongly dependent onpHvalue as it governa the dissociation equilibriumIn the present study long term labscale and benchscale experimenta were conducted to investigate

the feasibility of the anaerobic treatment process of wastewater from a pule and viscose fibre

manufacture In particular the aim of the investigations was to gain fundamental information about

process stability HzS inhibition and competition between SRB and MB on the available substrate leadingto the optimization of the operational conditions for largescale implementation

Methods

Labscale experiments

Three mesophilic up flow sludge bed labscale reactora 4 litres volume were operated in parallel over

a period of four years treating wastewater from a viscose fibre industry A second wastewater stream from

pule production was used as supplemental COD source The pHvalue in the influent was increased to 8

85 using an alkaline rich wastewater stream also from the viscose production COD sources in the

wastewater consisted mainly of acetic acid 2030 of CODt methanol 1015 of CODt hemi

cellulose and lignocellulosic compounds As theSCODratio in the wastewater shows table 1 COD

availability was not sufficient for a complete reduction of Sulphate stoichiometrical value 05Ammonium and phosphate were added to prevent nutrient limitation

Table 1 Influent wastewater specifications

Parameter Concentration Parameter Concentration

mgA mgACOD 45005500 Sulphate as S 30003500TOC 16001900 SCOD 0607

CODTOC 2829 Zinc 150250

Anaerobically digested sewage sludge from a municipal wastewater treatment plant was used as

inoculum Process temperature was kept at 38C Two reactora 1 and 2 were additionally equipped with

a gas scrnbbing system CuSO4 in order to reduce the HzS Concentration in the gas phase respectively to

1 and 5 vol One part of the desulphurised gas was reused in the reactor as stripping medium A third

reactor 3 was operated without removal of HzS resulting in concentrations of 10 to 15 vol The COD

loading per unit reactor volume Bco was increased stepavise from 075 to 2 kgCODm3dduring the

experimental period In addition one reactor was exposed to higherBco up to 55 kg CODm3dCOD

loading per unit of volatile suspended solidaXco varied between 02 and 28 kg CODkgVSSd

Benchscale experiments

By benchscale experiments on site of the factory with omine wastewater supply a different process

configuration was applied Figure 2 A mesophilic 36C anaerobic continuously stirred tank reactor

CSTR with external settling tank was coupled with a microaerophilic stage for biological Sulphideremoval Biological Sulphide oxidation is based on the conversion of Sulphide to elemental sulphur by the

colourless sulphur bacteria eg genera Thiobacillus

HZSzO2SH2O

Sulphide complete oxidation to Sulphate can be minimized by imposing proper operating conditions such

as oxygen supply and Sulphide loading rate Buisman et al 1990 One part of the HzSfree effluent

water stream was recycled to the anaerobic reactor to reduce HzS Concentration 2 to 4 vol Zinc in

influent wastewater was removed as zinc Sulphide before entering the CSTR using a part of the effluent of

the anaerobic settler The new process configuration for full scale application was developed together

with Paques BV wwwpaquesnl Influent wastewater composition was the same as for the lab scale

experimenta Bco varied between 2 and 55 kgCODm3dBXco between 02 and 05 kgCODkgVSSd

HzSrich water recicling loop

to Biogas

Storagetank

MikroAnaerobic

aerophilicreactor tank

ial3301 5001

Effluent to existingwasteater treatment plant

I Settling Settling

Itank tank I

Zinc sludgeI y

6501 650 1I

Air

IMixingtank erobic slutlge

ero

Çic slutlgeI

1151I rcwation wation

I II Excess slutlge I

I Sulfur sludge Ipulp productionwastewater stream I HzSfree water recicling loop IIJ

Figure 2 Flow sheet of the benchscale treatment plant

Analytical Methods

All analyses were carried out according to German Standard Methods for water wastewater and sludgeanalysis DII Effluent samples were filtered through paper filters white ribbon and stored at 4C until

analysis Samples for total sulphide HzS HS and SZ were collected with pipettes directly from the

reactora and immediately displaced in the reagent solution Methyleneblue method Concentration of

HzS and carbon dioxide COZ in the biogas was determined using shortterm test tubes Drger Duringbenchscale experimenta gas composition was analysed additionally by gas chromatography

Results and Discussion

COD remova efficiency and sulphide inhibition

Figure 3 shows the COD remova efficiency of the three labscale reactora calculated using mass balances

for experimental periods with steady state conditions COD and sulphate mass balances were applied to

check the reliability of the analytical resulta COD remova was referred to the corresponding BXcoin order to make resulta comparable In this regards it has to be noted that also the applicability of process

parameter related to VSS is in general restricted by the unknown amount of active biomass in the reactor

It can vary consistently under the different HzS concentrations appliedDuring the labscale experimenta the anacrobic process was stable at HzS concentrations in the gas

phase 1 vol 30 mgHZSfree1 in the liquid phase and reached a maximum remova efficiency of 70

at BXco of 04 kgCODkgVSSd About 80 of the residual COD in the effluent consisted of acetic acid

500700 mg CH3000H1 The pH value was about 72 COD remova efficiency decreased with

increasing loading rate as well as with higher HzS concentrations Figure 3 Inhibition of COD remova

showed a good correlation with the concentration of the unionized HZS which is in equilibrium with the

HzS concentration in the gas phase At HzS concentrations higher than 3 80 mg HZSfree1 the process

was characterized by an increasing intrinsic instability in the forni of accumulation of acetic acid10001500 mg CH3000H1 As a consequence the pHvalue in the process decreased below 7 thus

increasing the inhibiting effect of HZS and of acetic acid

SRB and MB present in the seed sludge were similarly inhibited by HZS concentrations between 3 and

10 vol see next paragraph figure 4a Acetotrophic microorganisms turned out to be strongly affected

by the inhibition The higher sensitiveness of acetate degradation compared eg to hydrogen uptake has

been already reported by previous studies OFlaherry et al 1998 and can be related to the lower

specific growth rate of acetotrophic microorganisms Gujer Zehnder 1983 50inhibition of the

COD remova was reached at a HZS concentration in the gas phase of about 9 vol 230 mg HZSfree1

which agrees well with values for mixed cultores and flocculent sludge reported by OFlaherryColleran 2000 According to Parkin Speece 1982 process inhibition in anacrobic reactors can be

counterbalanced by an increase of sludge age This was confirmed during the further experimenta in

benchscale Parravicini 2005

1 ooro

soro

G80

70

E50

40

U30n Reactor1 1 H2S

20 Reactor 2 5 H2S

10Reactor 3 10 H2S

L Benchscale reactor 24 H2S0

00 03 05 08 10 13 15 18 20 23 25 28 30 33 35 38 40

Bxcoo 9CODgVSSd

Figure 3 COD remova in function of the influent Bx coo for the lab and benchscale experiments

During the benchscale experiments process efficiency could be increased through the daily dosage of

thickened activated excess sludge TAS of the annexed industrial wastewater treatment plant dosage03 kgCODgiem3d TAS acted as COD and trace elements source seed sludge and flocculation agentthus improving sludge retention in the system COD remova at BxCOD between 03 to 05

kgCODkgVSSd and H2S concentrations between 2 and 4 vol averaged 85 A complete acetate

conversion was achieved under these conditions About 65 of the VSS in TAS were anacrobicallydegraded The result agrees with the usual VSS remova during anacrobic digestion of sewage sludge

B in the startup phase could be gradually raised from 2 to 55 kgCODm3dby daily increases of

24 and 13 respectively with or without TAS dosage The observed growth rate with TAS dosagewas therefore slightly higher than the values observed in similar anaerobic reactors Kroiss Svardal

1988

Substrate competition between SRB and MB

competition between SRB and MB on the available COD was evaluated using the ratio between

reduced sulphate as S and removed COD SCODA Substrate competition was intensive in a HzS

range between 1 and 10 vol In this concentration interval even after four years of operation MB were

still responsible for the reduction of 30 of the removed CODSCSB035 Figure 4a and 4b

os

os5

oe

oas

0 403

U p

vi 025

02Reactor1

015Reactor 2

01 Reactor 3

005 Benchscale reactor

0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

HzS vol

oso

os5

oso

oas a

oao

Ó 035 a

OU 030

X025N

020

015 tReadorl 1 H2S volk

010 Reador 2 5 H2S volk

005 Reador 3 10 H2S volk000

Time d

a b

Figure 4 SCSB ratio calculated out of mass balances for the three lab scale reactors for different

experimental periods in function of the H2S concentration in the gas phase a and of the time b

A good correlation was found between the methane production and the methanol fraction in the

influent Parravicini 2005 This confirma experimental resulta of previous studies on mesophilicdegradation of methanol in sulfidogenic anacrobic reactors Weijma 2000 MB could not adapt to higherH2S concentrations During the labscale experiments at H2S 10 vol complete sulfidogenic conditions

could be established after three years of operation Figure 4b The SCSB ratio slowly rose to04505

However under these extreme conditions sulphate remova efficiency and stability was strongly affected

by HZS inhibition

50 of the influent sulphate load was reduced under optimised process conditions benchscaleexperimenta For higher remova efficiency supplemental COD source is required Among the substratos

tested as externa COD sources ethanol and sucrose were found to be appropriate for sulphate reduction

The suitability of ethanol and sucrose as substrato for SRB has been already reported by previous studies

eg Greben et al 2000 Municipal anaerobically digested sewage sludge showed to be a suitable

inoculam for sulphate reduction in labscale as well as in benchscale experimenta Sulphate reduction

accounted already after two weeks operation for 50 of the COD remova

Impacts of anaerobic sulphate remova at the existing activated sludge treatment plant

Since 1987 the wastewater of the pule and viscoso fibre industry has been treated biologically in the

industrial wastewater treatment plant on sito based on a two afago activated sludge process The specificeffluent of the pulp and viscoso fibre production is 96 m3 per ton of produced viscoso The specific COD

load accounts for 108 kgton viscoso Zinc is removed from the wastewater as zinc hydroxide with lime

dosage Emitted sulphate load is reduced below regulation limita by dosing lime for gypsum precipitationDue to an increase of the pule and viscoso fibre production capacity the manufacture had to face the

problem of additional sulphate COD and zinc loads in the wastewater After severa years of

investigations in labscale and benchscale finally anaerobic biological treatment of viscoso wastewater

for sulphate remova was selected for alargoscale installation January 2004 the largoscale treatment

plant was ready for startup The selected process configuration consista of a combination of UASB

reactors followed by a microaerophilic reactor for biological sulphide oxidation Paques BVThe introduction of the new plant for sulphate remova in the existing wastewater treatment plant has

following main impacts considering the whole operational costa of the planaThe degradation of the additional COD load by SRB prevents the existing activated sludge plant from

overloading An enlargement of the plant is therefore no longer necessary Furthermore lesa excess

sludge is produced during anaerobic wastewater treatment duo to the lower yield of anaerobic micro

organisms The remova of zinc as zinc sulphide redutos significantly the amount of inorganic sludge to

be disposed However if sulphur sludge withdrawn in the aerophilic afago is also considered overall

sludge reduction is lesa significant Possibilities for reusing the sulphur sludge in future would allow for a

reduction in sludge to be disposed of

The remova of COD under anaerobic conditions allows energy savings by the aeration in the

activated sludge tanks The advantage is however consistently lower if the additional operational costa of

the new plant are considered eg heating pumping etc The costbenefit balance would get more

efficient if zinc was reused during the viscoso production Recycling sulphur for sulphuric acid

production is currently not profitable duo to low prices of the row stock

Technological aspects of anaerobic biological sulphate remova in viscoso wastewater

There are some particular aspects that should be considered while designing technologies for anaerobic

treatment of viscoso wastewater

According to the experimental resulta inhibition of SRB by HZS can be counterbalanced to some

extent by an increase of sludge age as dictated by the well known relationship between bacterial growthrate decay and sulphate effluent concentration This aspect should be taken finto account while choosingthe configuration of the anaerobic reactor Parkin Speece 1982 Besides an adequate largo reactor

volume also suitable sludge retention efficiency has to be ensured For the same reason precipitation of

zinc sulphide in apretreatment afago prior to the anaerobic reactor should be preferred The increased

amount of inorganic sludge in the anaerobic reactor can result in a drastic reduction of the sludge age duo

to the necessary higher surplus sludge withdrawnSrner 1986Severa literature sources report experientes in labscale as well as in benchscale with mesophilic as

well as with thermophilic anaerobic reactora for sulphate remova The choice of the process temperature

mainly dependa on wastewater influent temperature and specifications Amesophilic process is preferablewhen content of calcium ions in the wastewater is high duo to eg gypsumpreprecipitation treatment

Anaerobic thermophilic processes are known to be more sensible to temperature changes Bischofsbergeret al 2005 This would also increase the numbers of parameters needed to be controlled for stable

process conditions Additionally corrosion becomes a more acate problem at higher temperaturesBecause of the much higher solubility of HZS and COZ in water compared to methane lesa gas is set

free for mixing during sulphate reduction compared to fully methanogenic reactora This can leal to

decreased substrato mass transfer efficiency and therefore needs to be considered by selecting propor

reactor technology Cens et al 2000 In most cases the required mixing has to be ensured throughsupplemental mechanical devices eg recirculation loop mixer gas injection Values for specific energy

density between 2 and 6 Wattm3 reactor volume are considered to be sufficient for a good mixing in

anaerobic reactors Bischofsberger et al 2005Experimental resulta showed that for stable process operation HZS concentration in the gas phase in

the anaerobic reactor must be kept below 3 vol HzS removal can proceed either a in the gas phase or

b in the liquid phase Figure la and lba The first option enables the production of higher quality sulphur in respect of organic and inorganic

residuals through the uncoupling of wastewater and gas treatment However due to the low gas

production by sulphate reduction an extra carrier gas eg NZ is required for efficient HzS strippingAdditionally losses of COZ within the stripping step must be minimised to avoid disturbances in the

alkalinity equilibrium van Groenestijn et al 1995 Several wellestablished physicaUchemical or

biological technologies are available on the market for the removal of HzS in gas streams Janssen et al2000 The application of the processes mainly dependa on the amount of HzS to be treated In the case

studied biological sulphide oxidation and Claus process were considered among others for large scale

implementationb The removal of dissolved sulphide in the effluent of the anaerobic reactor offers the advantage of

an easier operation in exchange for lesa valuable sulphur sludge For this application biological sulphideoxidation to elemental sulphur was successfully developed as a costeffective method for the removal of

sulphur from wastewater streams compared to chemical processes Janssen et al 2000 Lens et al2000 It must be considered that the higher alkalinity present in the HZSfree effluent recycle loop will

raise the pHvalue in the anaerobic reactor This will exert on one hand a positive effect on HZS and acetic

acid inhibition on the other hand may result in higher precipitation of calcium carbonate Additionally it

must be ensured that sludge retention efficiency will not be affected by the higher hydraulic load appliedleading to a considerable loas of biomass in the anaerobic reactor with consequent depletion of the sludgeage Kroiss 1985

Viscose wastewater is poor in nutrients Nitrogen and phosphorous were added to the wastewater for

efficient treatment At a specific load BXco between 03 and 05 gCODgVSSd the CODN ratio for

nitrogen consumption in the pilot scale was in average 10007 The ratio agrees well with the values givenby Henze Harremoes 1983 for methanogenic reactors at a similar organic loading A phosphor dosageof CODP10002 was found appropriate Lack of micronutrients was not observed within the

experimenta As already mentioned the addition of TAS to the anaerobic reactor also accounted for a

sufficient pool of trace element

Conclusions

Aaerobic biological treatment showed to be a feasible option to treat wastewater from the viscose

industry for sulphate COD and zinc removal In this way it was also possible to prevent overloading of

the existing wastewater treatment plant and avoid its enlargements Furthermore sulphate reduction

opens new challenges for the reuse of zinc and sulphur in the viscose productionExperimental resulta showed that for high process efficiency the concentration of the produced HZS in

the gas phase in the anaerobic reactors must be kept below 3 vol 80 mg HZSfree1 Sludge age pHvalue and acetic acid concentration are further key parameters for stable operation The COD in the

wastewater was easily biodegradable 850 and a suitable substrate for sulphate reduction By HZSconcentrations ranging from 1 to 10 vol about 70 of the degraded COD was used by SRB On the

contrary methanol in the influent was found to be primarily a substrate for mesophilic MB Methane

production ceased only at HZS concentrations 10 vol Sulphate reduction accounted for 50and was

COD limited

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